Motor vehicles with driven axle independent suspensions include a pair of axle shafts (also referred to as split axles or half shafts), one for each wheel, as described, merely by way of exemplification, in U.S. Pat. No. 4,699,235 issued on Oct. 13, 1987 to Anderson and assigned to the assignee of the present patent application, the disclosure of which is hereby incorporated herein by reference.
Referring now to FIG. 1, the split axle drive system of U.S. Pat. No. 4,699,235 will be briefly described for point of reference, it being understood the present invention may apply to two wheel drive or four wheel drive systems.
Shown is a schematic plan view of a part-time four-wheel drive vehicle, comprising an internal combustion engine 10, transmission 12 and transfer case 14 mounted on a vehicle chassis (not shown). The engine 10 and transmission 12 are well-known components as is the transfer case 14 which typically has an input shaft (not shown), a main output shaft 16 and an auxiliary output shaft 18. The main output shaft 16 is drive connected to the input shaft in the transfer case 14 and is customarily aligned with it. The auxiliary output shaft 18 is drive connectable to the input shaft by a clutch or the like in the transfer case 14 and customarily offset from it. The transfer case clutch is actuated by a suitable selector mechanism (not shown) which is generally remotely controlled by the vehicle driver.
The main output shaft 16 is drivingly connected to a rear propeller shaft 20 which in turn is drivingly connected to a rear differential 22. The rear differential 22 drives the rear wheels 24 through split axle parts in a well-known manner. The auxiliary output shaft 18 is drivingly connected to a front propeller shaft 26 which in turn is drivingly connected to a split axle drive mechanism 28 for selectively driving the front wheels 30 through split axle parts. The split axle drive mechanism 28 is attached to the vehicle chassis by means including a bracket 71 on an extension tube 66.
Suitable split axle parts, commonly referred to as half shafts, are well-known from front wheel drive automobiles. These may be used for connecting the split axle drive mechanism 28 to the front wheels 30. The drawings schematically illustrate a common type of half shaft for driving connection to independently suspended steerable vehicle wheels comprising an axle shaft 76 having a plunging universal joint 78 at its inboard end adapted for connection to an output such as the flange 72 or 74 and the well-known Rzeppa-type universal joint 80 at its outboard end adapted to be connected to the vehicle wheel 30.
Problematically, axle shafts frequently exhibit “powerhop” when a large amount of torque is applied thereto. Powerhop typically occurs when tire friction with respect to a road surface is periodically exceeded by low frequency (i.e., below about 20 Hz) oscillations in torsional windup of the axle shafts. Powerhop produces oscillatory feedback to suspension and driveline components and can be felt by the vehicle occupants, who may describe the sensation as “bucking,” “banging,” “kicking” or “hopping.”
Axle shafts are typically manufactured from steel bar material and, as such, act as very efficient torsional springs. In the interest of reducing unwanted oscillations in the axle shafts, the standard practice has been to adjust the size (i.e., increasing the diameter) of the axle shafts in order to tune the resonating frequencies in such a way to minimize the negative impact of oscillations by increasing the overall torsional stiffness of the axle shafts, thereby reducing powerhop. However, increasing the diameter of the axle shafts results in additional packaging, mass and cost related problems, while not really addressing the core issue of directly damping oscillations that are associated with powerhop, to wit: lack of damping to absorb energy placed into the driveline by the negative damping characteristics of the tires during hard longitudinal acceleration or deceleration.
Accordingly, there is a clearly felt need in the art for axle shafts which are inherently damped very near the source of the oscillation, and thereby provide reduction of powerhop and associated driveline disturbances, such as for example axle shutter.